console
function test(){
var num=document.getElementById("in").value;
var type=document.getElementById("title");
var tynum,to;
for(var i=0;i<type.length;i++)
{
if(type[i].selected)
tynum=parseInt(type[i].value);
}
switch(tynum)
{
case(1):to=parseInt(num).toString(2);break;
case(4):to=parseInt(num,2);break;
}
if(isNaN(to))
to="输入非法字符了哦"
document.getElementById("out").value=to;
}
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
</head>
<body>
<div class="top" style="height:70px;border-bottom:1px solid #ddd;">
<img class="logoIndex" height="60" src="https://www.zenitour.com/img/logoIndex.png" style="margin-left:10px;margin-top:5px;">
</div>
<div class="content" style="text-align:center;margin-top:50px;height: 78%;">
<select name="title" id="title" style="width:152px;">
<option value="1">十进制转二进制</option>
<option value="4">二进制转十进制</option>
</select><br />
<input type="text" id="in" /><br>
<input type="text" id="out" /><br/>
<input type="button" value="转换" onclick="test()" />
</div>
<div id="bottom" style="text-align:center;border-top:1px solid #ddd;">
<span class="centerspan" style="font-size:10px">Copyright © 2017 Zenitour lnc All Rights Reserved 版权所有-哲途教育 渝ICP备13002955号-2</span>
</div>
</body>
</html>
html,body {
width: 100%;
height: 100%;
margin: 0;
padding: 0;
}